During an early episode of the über-pyrotechnic television series MythBusters, Adam Savage was busted by the camera crew for misremembering his predictions of the probability of an axle being ripped out of a car, à la American Graffiti. When confronted with the unmistakable video evidence of his error, Adam sardonically rejoined: “I reject your reality and substitute my own.”

Skepticism is the fine art and technical science of understanding why rejecting everyone else’s reality and substituting your own almost always results in a failed belief system. Where in the brain do such belief processes unfold? To find out, neuroscientists Sam Harris, Sameer A. Sheth and Mark S. Cohen employed functional magnetic resonance imaging to scan the brains of 14 adults at the University of California, Los Angeles, Brain Mapping Center. The researchers presented the subjects with a series of statements designed to be plainly true, false or undecidable. In response, the volunteers were to press a button indicating their belief, disbelief or uncertainty. For example:

Factual:
Most people have 10 fingers and 10 toes.
Eagles are common pets.
The Dow Jones Industrial Average rose 1.2% last Tuesday.

Ethical:
It is bad to take pleasure at another’s suffering.
Children should have no rights until they can vote.
It is better to lie to a child than to an adult.

The findings were revealing. First, there were significant reaction time differences in evaluating statements; responses to belief statements were significantly shorter than responses to both disbelief and uncertainty statements (but no difference was detected between disbelief and uncertainty statements). Second, contrasting belief and disbelief in the brain scans yielded a spike in neural activity in the ventromedial prefrontal cortex, associated with decision making and learning in the context of rewards. Third, contrasting disbelief and belief showed increased brain response in the left inferior frontal gyrus, the anterior insula and the dorsal anterior cingulate, all associated with responses to negative stimuli, pain perception and disgust. Finally, contrasting uncertainty with both belief and disbelief revealed elevated neural action in the anterior cingulate cortex, a region associated with conflict resolution.

What do these results tell us? “Several psychological studies appear to support [17th-century Dutch philosopher Benedict] Spinoza’s conjecture that the mere comprehension of a statement entails the tacit acceptance of its being true, whereas disbelief requires a subsequent process of rejection,” report Harris and his collaborators on the study in their paper, published in the December 2007 Annals of Neurology. “Understanding a proposition may be analogous to perceiving an object in physical space: We seem to accept appearances as reality until they prove otherwise.” So subjects assessed true statements as believable faster than they judged them as unbelievable or undecidable. Further, because the brain appears to process false or uncertain statements in regions linked to pain and disgust, especially in judging tastes and odors, this study gives new meaning to a claim passing the “taste test” or the “smell test.”

As for the neural correlates of belief and skepticism, the ventromedial prefrontal cortex is instrumental in linking higher-order cognitive factual evaluations with lower-order emotional response associations, and it does so in evaluating all types of claims. Thus, the assessment of the ethical statements showed a similar pattern of neural activation, as did the evaluation of the mathematical and factual statements. People with damage in this area have a difficult time feeling an emotional difference between good and bad decisions, and they are susceptible to confabulation—mixing true and false memories and conflating reality with fantasy.

This research supports Spinoza’s conjecture that most people have a low tolerance for ambiguity and that belief comes quickly and naturally, whereas skepticism is slow and unnatural. The scientific principle of the null hypothesis—that a claim is untrue unless proved otherwise—runs counter to our natural tendency to accept as true what we can comprehend quickly. Given the chance, most of us would like to invoke Adam’s Maxim because it is faster and feels better. Thus, it is that we should reward skepticism and disbelief and champion those willing to change their mind in the teeth of new evidence.

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